The Global Positioning System (GPS) is a U.S. government-supported system for precise position and velocity determination of objects. Orbiting satellites emit coded radio frequency signals that are received and processed by receivers on or near the surface of the earth to obtain pseudorange measurements, approximate instantaneous distances between the satellites and receiver. Because the satellite orbits and signal transmission times are known precisely, the receipt time of a particular signal bit can be used to quantify the transit time or range to the particular satellite. The orbits of the GPS satellites are arranged in multiple planes so that signals can be received from at least four satellites at any point on or near the earth, allowing precise position and velocity measurements of the receiver. Each satellite continually transmits a spread-spectrum signal that is modulated by a pseudo-random number (PRN) code unique to the satellite. A receiver can therefore identify and separate signals from each satellite into separate channels, process the channel data separately, and combine the processed data to compute a position of the receiver. In addition to the PRN code, the signals are modulated by slower-varying data signals defining the satellite orbits and other relevant information needed for the computations. The most common PRN code is a binary sequence of 0's and 1's or −1's and +1's that modulates the carrier phase.
The PRN code is combined with a 50 Hz data stream using binary phase shift keying (BPSK), and thus the polarity of the PRN code potentially changes every 20 ms, i.e., a 180° phase shift in the PRN code can occur every 20 ms.
In the receiver, local signals corresponding to known PRN codes are generated and correlated with the received signals to detect PRN codes in the received signals. Since the time at which each bit of a known PRN code sequence is transmitted from the satellite is known, the time of receipt of each bit is a direct measure of the transmit time of the signal from the satellite to the receiver, and therefore a measure of the distance between the two. Based on the computed relative phase of the received signal, the receiver calculates the desired quantities of distance, velocity, etc.